(1) Field of the Invention
The present invention relates to a direct methanol fuel cell, and more detailedly relates to a compact direct methanol fuel cell that is suitable for power sources of portable electric appliances such as cellular phones, laptop computers, and the like.
(2) Description of the Prior Art
Generally, a fuel cell is a battery which is comprised of a fuel cell unit made up of an air electrode layer, an electrolyte layer and a fuel electrode layer formed into layers, a fuel feeder for supplying fuel as a reducer to the fuel electrode layer, and an air feeder for supplying air as an oxidizer to the air electrode layer, and causes electrochemical reactions involving the fuel and oxygen in the air within the fuel cell unit, so as to produce electric power for an external circuit. Recently, various types of fuel cells have been developed.
Because of recent increase in awareness of environmental issues and energy saving, fuel cells as clean energy sources have been investigated for various purposes. In particular, direct methanol fuel cells, as disclosed in Japanese Patent Application Laid-open Hei 5 No. 258760, Japanese Patent Application Laid-open Hei 5 No. 307970, which are capable of generating electricity by direct supply of a liquid fuel containing methanol and water, have attracted a great deal of attention.
Of these, liquid fuel cells that utilize capillary action to supply a liquid fuel have been disclosed by Japanese Patent Application Laid-open Sho 59 No. 66066, Japanese Patent Application Laid-open Hei 6 No. 188008, and others.
Since these liquid fuel cells supply the liquid fuel from the fuel tank to the fuel pole by capillary force, no pump for pumping the liquid fuel is needed, resultantly producing an advantage for miniaturization. It is true that this type of liquid fuel cell which utilizes capillary force only is suitable for compact configurations, but has the following disadvantage. That is, since fuel is supplied in its free state to the fuel pole, a long period use of a portable appliance with a cell under the usage environment in which the cell unit is always moved back and forth and left and right or turned upside down, makes it impossible for the fuel to flow correctly, causing problems such as fuel supply deficiency, air bubble entrance to the tank, and the like, or inhibiting a fixed amount of fuel supply to the fuel rod.
As one of countermeasures against these defects, Japanese Patent Application Laid-open 2001-102069, for example, discloses a system in which liquid fuel, after it is introduced into the cell by capillary force, is used by evaporating it within a fuel vaporizing layer. However, this method still cannot solve the basic problem, i.e., the insufficient fuel followability problem. Further, because of use of fuel after its evaporation from liquid, the fuel cell of this configuration produces difficulties in miniaturization.
In this way, under the current situation, the conventional direct methanol fuel cells suffer the problem that the output power during operation may fluctuate due to instability of the fuel supply in supplying the liquid fuel directly to the fuel pole, and face difficulties in achieving the level of miniaturization needed to be built into a portable appliance whilst keeping stable characteristics.